(a) Field of the Invention
The present invention relates to a heat resistant adhesive composition which is extremely excellent in heat resistance and further, has flexibility, and to a method of bonding two material bodies by using it. Particularly, the present invention relates to a heat resistant adhesive composition which is useful particularly for production of flexible printed boards and multilayer flexible printed boards, and to a method of bonding two material bodies using the heat resistant adhesive composition, the method being particularly suitable for the production of these flexible printed boards.
(b) Description of the Related Art
Flexible printed boards generally have been produced by attaching a sheet of metal foil to a polyimide film as a base film using pressure with an adhesive being disposed between them, then removing unnecessary portions of the metal foil to make a printed circuit, and finally protecting the printed circuit with a coverlay film or a solder resist ink. However, the adhesives which have been mainly used for the method are epoxy adhesives or acrylic adhesives, which are inferior in heat resistance. Therefore, although polyimides, which have good heat resistance, have been used as the base film, the adhesives have characterized the properties of the flexible printed boards, and the excellent heat resistance of the polyimides could not be reflected sufficiently in the flexible printed boards. For instance, the flexible printed boards produced by using such adhesives have been of such a low soldering heat resistance as about 300.degree. C. There have been another problem that even if the flexible printed board had a bond strength of 1.5 kgf/cm at room temperature, it is reduced to 0.5 kgf/cm or less when temperature exceeded 100.degree. C.
In order to solve these problems, we have developed adhesives which have good heat resistance and flexibility and can be used as the adhesives for the production of flexible printed boards without causing substantial deterioration in the properties of the flexible printed boards, such as heat resistance. The adhesives however are expensive and require a high temperature higher than 250.degree. C. for thermopressing.
With regard to the method of producing one-sided metal-clad laminates, there has been developed a method called direct coating method, wherein a laminate is produced by directly coating a sheet of metal foil with a polyimide without using adhesives. However, this method cannot be used for producing double-sided flexible metal-clad laminates, for laminating coverlay films on printed boards or for laminating one-sided flexible metal-clad laminates or printed boards to produce double-sided flexible metal-clad laminates or multilayer flexible printed boards.